2567
LATTICE
and
where CY’describes the efficiency with which the electrons relax C uiu H (i,e., the so-called three-spin effect4). T l C H thus refers to C-H relaxation in the absence of electrons. Substituting eq 5 and 6 into eq 3 and writing4 l/TICE= k[E] and a’/TIHE= cy[E], eq 4 becomes r
1
1
E Figure I .
[E] is the concentration of electron spins and 1 1 - -~ Tic(LH) TicL
+
1 __-
TlCH
Equation 7 thus describes the effect of electron spins on the 13COverhauser enhancement factor during proton irradiation. The effect of electron spins on the observed 13Cline width (Av,.) is given by Av,,?= [ T T ~ ~ ( L E H )where ]-~ 1 - 1 +- 1 6k[E] T,,(LEH) TICI, TSCH
.~
~
~~
H
+
+ CY[E] (8)
6 is the ratio Tl/T?for the C--E coupling and is normally a little greater than unity6 since dynamic scalar coupling operates between 13Cnuclei and electrons.’ Substitution of the condition 4yH2H22 >> ir2JCH2for complete proton decouplingYinto eq 2 and evaluation of (TlT2)Eciu eq 6 and 8 as outlined for 13Cshows that decoupling always implies SH = 1 unless JCH is very small (-3 Hz). Equations 7 and 8 predict the following significant points. (1) Addition of increasing amounts of a paramagnetic species reduces (or increases) the 13C Overhauser enhancement to a constant value before significant broadening of the 13C line occurs. This is true for a wide range of k values. (2) When (k a)Tlc. (LH)[E] >> 1 the enhancement facto1 is 1.988SHcu/(k CY). Thus, when cy = 0 (which commonly occursB~*), the Overhauser effect is completely eliminated. (3) The initial enhancement factor ([E] = 0) is 1.988SHTlc. (LH)/TiCH as for the two-spin case.4 (4) Absolute values of k, cy, Tlc(LH), and TlCH (and thus TlcL) can be obtained by fitting eq 7 to a plot of (C, - C,)/C, us. [El unless T ~ C H= TlC(LH) or k = 0. (5) k 0: rCE-3, where rCE;is the 13C-E distance. For a given value of [E], therefore, the Overhauser enhancements of those 13C nuclei closest to the paramagnetic species will, in general, be preferentially reduced. This effect is observed at values of [E] at least two orders of magnitude lower than the observation of preferential line broadening.g The validity of the theory is indicated by the fact that La Mar’s esperiniental results for p-dioxane3 are de-
+
+
(6) R . A . Dwek, J . G. Kenworthy, D . F. s. Natusch, and R. E. Richards, Mol. Phj,s., 13, 27 (1967). (7) D. F. S . Natusch and R . E. Richards, Chem. Com/nuti., 579 (1966). (8) J. A . Poplc, W . G .Schneider, and H. J . Bernstein, “High Resolution Nuclear Magnetic Resonance,” McGraw-Hill, New York, N. Y., 1959. (9) R. Mathur and N. C. Li,J. Amer. Chem. Soc., 86,1289 (1964)
I3Crelaxation processes in a three-spin system.
scribed quantitatively by eq 7 and 8 if account is taken of the oxygen dissolved in his samples. The effects of dissolved oxygen, complex formation, and radical dimerization will be discussed elsewhere. The theory suggests that any soluble, nonreactive paramagnetic species will reduce the Overhauser enhancement almost to zero when CY is small. Preference can therefore be given to readily available, stable, easily handled materials. In cases where CY is appreciable its value (and thus C,) can be obtained by curve fitting. It appears feasible, therefore, t o obtain the number of 13Cnuclei contributing to a given resonance by addition of a paramagnetic species to the sample. In addition the preferential reduction of 13C Overhauser enhancements can be used to establish the binding sites of paramagnetic metal ions and spin labels in labile complexes. Acknowledgment. The author wishes to thank Dr. G . N. La Mar for suggesting the problem and Professor H. S. Gutowsky and Dr. D. K. Dalling for helpful discussions. The support of the National Science Foundation and the U. S. Office of Naval Research is gratefully acknowledged. David F. S . Natusch School of Chemical Sciences, Unicersity of Illinois Urbana, Illinois 61801 Receiced Jarirrary 21, I971
The Mechanism of the Chlorination of Anilines and Related Aromatic Amines. The Involvement of Nitrenium Ions’ Sir: The careful work of Neale and coworkers2 and of Haberfield and Paul3 has firmly established the intermediacy of N-chloramines in the chlorination of aromatic amines by reagents such as N-chlorosuccinimide and calcium hypochlorite. Although there is little doubt about the initial step in the reaction being N(1) Paper XVI in a series on the Chemistry of Nitrenium Ions. For previous papers in this series, see P. G . Gassman and A. Carrasquillo, Tetrahedron Lett., 109 (1971), and P. G. Gassman and J . H. Dygos, ibid., 4749 (1970), and references therein. (2) R . S. Neale, R. G. Schepers, and M . R. Walsh, J . Org. Chem., 29, 3390 (1964). (3) P. Haberfield and D. Paul, J . Amer. Chem. SOC.,87,5502 (1965). (4) Various other chlorinating agents have been used. For additional synthetic applications, see T. A. Foglia and D. Swern, J . Org. Chem., 33, 4440 (1968); J . M. Muchowski, Can. J . Chem., 48, 422 (1970); and A . M. Pinchuk, L. N . Markovskii, and I. M . Kosinskaya, Zh. Obshch. Khim., 38, 1008 (1968).
Communications to the Editor
2568 Table I. Rates of Rearrangement of N-Chloroanilines in Ethanol Buffered with 0.1 N Acetic Acid-O.1 N Sodium Acetate Temp, 10.02"C
Compd
k , sec-' (5.27 i 0.05) X (2.37 i 0.01) x 10-4 (8.79 i 0.01) x 10-4
-10.0 0.0 10.0 25. 0Oa 20.0 30.0 40.0 25. 0Oa 30.0 40.0 50.0 25. OOa 40.0 50.0 60.0 25.W 70.0 80.0 90.0 25. ma 90.0 100.0 110.0 2 5 . OOz
5
6
7
8
9
10
AH*, kcal/mol
krel
x
5.85
*, eu
20.4
-0.3
21.2
-5.0
21.9
-6.6
23.0
-3.1
26.3
-4.9
28.4
-2.2
1,800,000
10-3
(7.47 f 0.01) x (2.60 i 0.01) x (8.18 i 0.04) X 1.40 x 10-4 (3.83 f 0.03) X (1.27 0.02) x (3.91 i 0.01) x 2.06 x (1.12 f 0.02) x (3.60 f 0.02) x (1.09 0.01) x 1.65 x 10-5 (1.07 i 0.02) x (3.97 + 0.02) x (9.35 i 0.32) x 2.96 x (2.04 i 0.08) X (6.10 f 0.04) X ( I .68 i 0.04) X 3.19 x 10-9
AS
10-5 10-4 44,000 10-4 10-4 6,500 10-4 10-4 10-3 5,200 10-5 10-5
9
I
Rates were determined by iodometric titration of active chlorine. Unreacted N-chloroanilines a Extrapolated from other temperatures. were first treated with acidic potassium iodide followed by titration with thiosulfate using a dead-stop titration method.
chlorination, the mechanistic pathway from the N chloramine to the halogenated aromatic ring remained a mystery. We now wish to report that this chlorination proceeds aia heterolytic cleavage of the N-Cl bond to yield a nitrenium ion and chloride anion.
i
-21\ -6
-4
-2
2
0
b
.6
7
.8
Ut
Figure 1. u+p plot for the solvolysis of N-chloro-N-terf-butylanilines in buffered ethanol.
In general, the reaction of aromatic amines with sources of positive halogen proceeds to give mixtures of ring-chlorinated products. As shown below for aniline derivatives, the ortho:para ratio is normally ca. 2 and the N-chloroaniline intermediate is generally not iso-
1
2
3
I c1 4
Journal o f t h e American Chemical Society / 93:IO
CI
c1
I
-4
9
6
I
10
lated or observed. Several mechanistic schemes could be suggested for the conversion of 2 into 3 and 4. Of the various possibilities, the most likely choices are heterolytic cleavage of the N-CI bond to yield amide anion and positive chlorine, or heterolytic cleavage of the chloramine to produce divalent electron-deficient nitrogen (nitrenium ion) and chloride anion. In order to distinguish between these divergent concepts, we have studied the kinetics of the facile rearrangement of substituted N-chloroanilines in ethanol buffered with 0.1 N acetic acid-0.1 N sodium acetatea5>'j Table I lists the rates of solvolysis of a series of substituted N-chloroanilines in buffered ethanol. Inspection of Table I shows that the presence of electron-with(5) All rates were followed to a t least 65z reaction. All of the rates measured in buffered ethanol showed excellent pseudo-first-order kinetics. (6) Investigation of the solvolyses of 5-10 in unbuffered ethanol showed that the buffer was unimportant for 5-8 since these four N chloramines showed essentially the same rates and same products, irrespective of the presence of buffer, However, in the absence of buffer, 9 and 10 gave kinetic data indicative of a rapidly increasing rate of reaction. I n view of the excellent pseudo-first-order rates obtained for 9 a n d 10 in buffered ethanol, the observations for the unbuffered solvolyses indicated that a change of mechanism was occurring. Clearly, the solvolysis of 9 and 10 could be acid catalyzed, while the solvolysis of 5-8 was not sensitive t o small amounts of acid. Orthochlorinated products were obtained from 9 and 10 in 99 and 9 6 % yields, respectively, under the unbuffered conditions.
/ May 19, I971
2569
drawing substituents results in a marked decrease in the rate of rearrangement of the six anilines studied. The overall rate decrease in going from the para methyl derivative, 5, to the para cyano case, 10, was 1.8 X lo6. This decrease in rate indicated that the transition state for the rearrangement of 2 involved the generation of a positive center adjacent to the aromatic ring. Confirmation of this concept was provided by Figure 1 which shows a plot of log K GS. the Brown c+ substituent c o n ~ t a n t s . ~For the six compounds studied, we obtained an excellent correlation with c+. The p for the reaction was -6.35 and the correlation coefficient was 0.996. Each of the compounds listed gave ortho chlorinated anilines a s the major product. When the para position of the aniline was blocked the yields of ortho chlorinated products ranged from 65 for 10 t o 85 % for 5. As previously noted,*z37 gave a mixture of ortho and para substitution. Two interesting facets of the product studies merit discussion. First, solvolysis of 5 in buffered ethanol gave 10% of l l a , indicating the presence of positive charge on the ring. When the
lla, R = CH2CHJ b,R=CHJ
12
solvolysis of 5 was carried out in pure unbuffered methanol, we obtained 3 2 x of l l b and 61 of the normal ortho chlorinated aniline. The increased amount of 2,5-cyclohexadienone derivative,811, formed in methanol was consistent with the difference in solvolytic properties between ethanol and methanol. The second interesting feature of the product studies was the amount of starting aniline, 12, which was formed in the reaction. The amount of this amine gradually increased as the substituents became more electron withdrawing, going from 1 when X was methyl to 29 % when X was cyano. This indicated that the presence of electron-withdrawing substituents decreased the amount of charge delocalized into the aromatic nucleus. Increased positive charge on nitrogen appeared to result in increased amounts of 12.9 Investigation of the solvolysis of 5-10 in buffered ethanol showed that the mode of heterolytic cleavage involved the formation of an aryl nitrenium ion (anilenium ion) and chloride anion. The large p of -6.35 indicated that considerable delocalization of the charge onto the aromatic nucleus must have been present. This is consistent with the relative abilities of divalent nitrogen and trivalent carbon to bear a positive charge.1° (7) H . C . Bro-11 and Y. Okamoto, J . Amer. Chem. SOC.,80, 4979 (1958). (8) We have recently shown that the reaction of 5 with silver ion in methanol produces l l b in a 7 0 Z yield in a useful synthesis of 2,5-cyclohexadienone derivatives: P. G. Gassman and G. A. Campbell, Chem. Commrin., 427 (1970). (9) In these cases it is unknown whether 12 arises cia a hydride abstraction mechanism or riu spin inversion of the singlet nitrenium ion to a hydrogen-abstracting triplet nitreilium ion. For an example of the latter see P. G. Gassman and R . L. Cryberg, J . Amer. Chem. SOC.,91, 5176 (1969). (10) It is interesting to compare the solvolysis of derivatives of2 with that of substituted I-chloro-I-arylethanes. The p for the latter in 20:80 water-dioxane has been reported to be - 4.50 (C. Mechelynck-David and P. J. C. Fierens, Tetrahedron, 6 , 232 (1959)). I t would appear that the nitrogen cation was much more dependent o n charge delocalization into the aromatic ring than was the analogous carbonium ion.
The solvolysis of N-chloroanilines to give anilenium ions adds still another category to the growing list of reactions which occur uia nitrenium ion intermediates. l 1 Acknowledgment. We are indebted to the National Cancer Institute of the Public Health Service for a grant in support of this investigation. (11) For a recent review see P. G. Gassman, Accourits Chem. Res., 3, 26 (1970). (12) National Defense Education Act Fellow,, 1967-1970; StauKer Chemical Fellow, 1970-1971.
Paul G. Gassman,* Gerald A. Campbell12 Department of Cliemistry, The Ohin State Uiricrrsity ColrimbiiA, Oliin 43210 Receiced March 6 , 1971
Mechanism of Photochemical Ring Expansion Reactions of Cyclic Ketones'
Sir: A number of publications have appeared concerning the photochemical ring expansion reactions of selected cyclic In most cases, the rearrangement was proposed to involve homolysis of a carbon-carbon bond CY to the excited carbonyl function, followed by electronic rearrangement and rebonding to form an oxacarbene intermediate (eq 1). Evidence for
W
W 1
2
3
intermediate 3 derives from its trapping by alcoholic solvents to yield acetals as isolable products. Yates has suggestedg that ring expansion is associated with certain structural features which inhibit other competing intramolecular rearrangements; i.e., ring expansion only occurs from 2 when other reactions of this species are disfavored. We present results here which demonstrate that while certain structural features are necessary for observation of photochemical ring expansion, the reaction can compete favorably with other common fates of 2.'" Key compounds in this study are cyclopentanones 4, 6, 9, and 11, and cyclobutanones 15 and 17. The photoproducts, efficiencies, and reactivities of the former set are listed in Table I. Photochemical ring expansion of 6 is in itself a rather unusual reactionfi,ll (1) Molecular Photochemistry. XLIV. Paper XLIII : I
